How to send a message to a TThread from main thread in Delphi?

Question

I want to send a message to a thread and handle it in the thread. How can I do this in Delphi? I guess PostMessage is the way to go, but the examples I've seen so far are describing the other way, i.e. from the thread to main thread.

Answer 1

I won't even try and explain or write any code. Just look at this tutorial. It's a little old, but very good imho. Multithreading - The Delphi Way

Answer 2

You can either have a message loop (possibly with a hidden notification window) in your thread and send a Windows message to it, or you can use a more native (less-GUI) way of doing it, such as a queue protected by a critical section combined with a manual-reset event that the thread waits on and the sending thread signals.

A more general solution is a producer-consumer queue, which in the classic implementation uses a couple of semaphores to keep track of consumers and producers and a third semaphore for mutually exclusive access to the queue; however, more optimal producer-consumer queues are available on the net.

Answer 3

Why would you need to do it? It is only for one reason that I ever had to create a message loop in a secondary thread, and that is because the thread used COM objects. The calls to OleInitialize() and OleUnitialize() are a sign that you need a standard GetMessage() loop. In that case it's also necessary to just post messages to that thread, using PostThreadMessage(), because normal blocking synchronization calls would interfere with the message loop. Otherwise, just don't do it.

If you are at Delphi 2007 or 2009, be sure to look into OmniThreadLibrary by Primož Gabrijelčič, it should make your job much easier.